CAP1391 shows as bearingless in SkyDemon via PAW

[exfirepro]

Hi Alex,

Sorry for the delay in replying. I was away at Popham all weekend so rather busy, then had to fly all the way back up to East Fortune.

You still haven't given us your actual current PAW software version - it is reported on the first line of each track file, including the one you quote from in your latter post.

Thank you so so much! All super interesting and very details and triggered lots of thoughts, so might not be a short answer from myself either as some points are carry over from the original post idea and some completely new

1) The erratic behaviour you shared – does that mean that if I have mode-s 3D enabled – I am fairly likely to be displayed with similar output in my PAW? If so – I must admit I would probably go for disabling the 3D MLAT as got me thinking that I might be wrongly looking for a plane in a certain direction, so rather look all around due to a bearing less notification.

No definitely not - enabling Mode-S 3D simply allows you to see Multilaterated (triangulated) Positional Targets Reports for Mode-S aircraft where these are available, instead of just Bearingless Target Reports. It has no effect whatever on the performance of your own aircraft transmissions. My advice would always be to enable Mode-S 3D as soon as you are fully aware of what it does and how it works (which is why it isn't enabled by default).

2) I in fact flew today and experienced a situation when an aircraft was first showing as bearlingless target and then next minute as a specific position. My Mode S 3D was disabled then, so in such circumstances it would totally make sense that possibly that other aircraft had some portable device that was visible sometimes to my PAW but not always. See first screenshot attached.

Unfortunately the screenshots are a bit poor to see exactly what they are reporting (especially the one with the red ring). Even zooming in, I simply can't see enough to confirm whether the two reports are from the same aircraft.  Assuming, however that they are, that would be perfectly normal expected behaviour for a Mode-S aircraft if you had Mode-S 3D enabled. Mode-S displays as Bearingless (so a circle on SD with Relative Alt - and Reg ID if 'Show Registrations' is selected in SD Nav Options) when you have NO ATOM, iGRID or SkyGRID uplink. It then swaps automatically to a 'known position' target if you get a Mode-S 3D report from ATOM, iGRID or SkyGRID, then drops back to Bearingless if you lose the uplink(s) again. I note, however, that you say you had Mode-S 3D disabled. In that case the signal must have been swapping between raw Mode-S and CAP1391 (or PilotAware) - so between Bearingless and Known Position - which fits with what I found in my analysis of your earlier track reports for G-NXOE). PAW would then prioritise the 'known position' reports while they were being received (even from a weak CAP1391 or PAW signal) but would drop back to a Bearingless Report with no position whenever the CAP1391/PAW signal dropped out.

3) I have to point out that original question is unfortunately still remains a bit of a mystery because it was not really about MLAT or anything. I know for sure that they left a CAP1391 on by mistake in a parked and completely shut down aircraft and I was only 50m away from it, but still saw it as bearing less through PAW (second screenshot).

That is still a bit of a mystery.

The lower screenshot 'appears' to show G-NXOE reporting as 'Mode-S + ALT' - or we a.) wouldn't have a Bearingless ring, and b.) wouldn't have the Aircraft Reg showing. If it was only transmitting Modes A/C, it would report as 'C-xxxx'). The REG 'might' of course be coming from the SkyEcho, but I can't see how you would get a Bearingless ring associated with it - except as I explained earlier that there was a very short-term glitch in (I think) the Ground Station Software, which was reporting some CAP1391 units as DF17 - which is the mode transmitted by Mode-S/ES (ADSB) Transponders. That fault was however found and fixed extremely quickly, so shouldn't 't have persisted (hence why I need to know the PAW software version that was in use at the time of your log.

My log decoding skills are a bit rusty (I usually leave that to Lee), but your reported log-line certainly does seem to show G-NXOE reporting as Mode A/C - and not Mode-S or S/ES (ADSB), sorry I can't remember what the rest of it means. That might be explainable if the transponder had been left in 'Standby' or 'Ground Mode' or has a Squat Switch, to stop full transmission until in the air.

I still thing it's an issue with G-NXOE.

Best Regards

Peter

[a.alexeev.p]

Thank you so much for being so detailed an helpful once again :-) and hope the event went well!

So, the software version was 20230316

1) Apologies I probably sounded a bit confusing here. So it was nothing about transmission really... In the screenshot attached (hopefully readable) is playback of me flying GLOOC that only had Mode S transponder and sub sequentially MLATed by PAW atom network on the morning of 19th of April. The playback contains a big "jump" to the south, which did not occur in reality. So, if my friend was in another aircraft nearby with PAW and 3D Mode S enabled, then it sounds like s/he would see GLOOC quite far away from the actual position?

2) Thanks a lot! All makes sense. I was able to figure out from your playback system that that aircraft likely had Mode S + PAW, so what you explained totally makes sense and no questions on it.

3) GDBOD is another aircraft that was just next to GNXOE. It is a club aircraft that also normally has a CAP1391 onboard, also was parked and completely shut down, so I am very reluctant to agree that something was not right with the transponder as both aircraft were definitely off completely. I need to see if I can reproduce the issue myself, but I don't have a 1391 device available, so need to ask someone.

[steveu]

3) GDBOD is another aircraft that was just next to GNXOE. It is a club aircraft that also normally has a CAP1391 onboard, also was parked and completely shut down, so I am very reluctant to agree that something was not right with the transponder as both aircraft were definitely off completely. I need to see if I can reproduce the issue myself, but I don't have a 1391 device available, so need to ask someone.

The most common CAP1391 device is the SkyEcho 2, it is totally autonomous, and if not specifically shut down on the power button on the front of it, it will stay up and not go off with aircraft power. It will then stay up until the battery goes flat, 12 hours?

[a.alexeev.p]

Exactly right! So what I think the issue is that when aircraft is shut down - a SE2 likely to be left on. An easy thing to miss as I sometimes have forgotten to unplug power from my paw in the past

[a.alexeev.p]

So I attempted to replicate the issue… didn’t have much time, so I admit I did not see bearingless ring in SD, but I didn’t see the SE2 position either. On traffic page the aircraft of interest (GDBOD hex 40632C) was showing as CS only.

In the log found the following line:
$PALOG,20230516,162817,PAWRT,40632C,1,A,1,C,1,S,0,ES

Any more thoughts or suggestions of what I should try?

[Admin]

Can you please confirm if G-DBOD has a transponder
Also was the se2 detected by ANY receiver?

Also, how close were you to the unit during the test ?

Thx
Lee

[a.alexeev.p]

Hi Lee,

I just borrowed the SE2 from the aircraft owners for 5 minutes for my testing, so I suppose what the actual aircraft has is less relevant? But normally the aircraft is flown with a Mode S transponder + SE2.

The SE2 was visible on a flight tracking app - same as before with exact position and ADSB source confirmed. I placed it a few meters away i.e. my PAW was inside an aircraft and the SE2 on tip of the wing.

It was purely stationary ground test with no engines running or anything.

Regards,
Alex

[steveu]

Any more thoughts or suggestions of what I should try?

A skyecho has a vso setting
Anything less than this and it will not emit a position report
Can you check its setting

Does the CAP1391 have the setting for flying/not flying enabled? I think the default may be 10 knots to switch over? Been a while since I looked...

[a.alexeev.p]

I must admit I was in a rush and didn’t look at that to be absolutely sure, so will make sure to confirm. However, don’t see how it would show on a plane tracker if it wasn’t transmitting… hence I am quite sure setting is set to transmit always, but will check

[a.alexeev.p]

Ok, so…. This time I spent a good amount of time trying different settings of a SE2.

I tried:

1) transmit setting at 0 kts and at 10 kts
2) position SE2 within 2-3 meters of paw and within 10-15 meters of paw
3) Ensured that the actual aircraft was parked and OFF

This time I did not manage to replicate the bearingless target issue in any configuration…in fact, initially SE2 signal did not show at all. After a couple of reboots it did show on SD via PAW with either transmit setting and distance from PAW.

Very inconclusive tests to be honest… the only thing that remained consistent was that SE2 was always visible on a flight tracking app (even when not visible on PAW). I know for sure that in principle (and probably in most cases) I do see SE2 from flying aircraft. But what happened here will still remain a mystery.

[exfirepro]

Hi again Alex,

Thanks for the further update.

Ok, so…. This time I spent a good amount of time trying different settings of a SE2.

I tried:

1) transmit setting at 0 kts and at 10 kts
2) position SE2 within 2-3 meters of paw and within 10-15 meters of paw
3) Ensured that the actual aircraft was parked and OFF

This time I did not manage to replicate the bearingless target issue in any configuration…in fact, initially SE2 signal did not show at all. After a couple of reboots it did show on SD via PAW with either transmit setting and distance from PAW.

SE2 shouldn't show on PAW or the flight tracking app if its Tx setting was set (and saved) at 10Kts, unless the unit was moving at more than that threshold speed. If as you say, it was reporting on SD via PAW (and on the flight tracking app) after a couple of reboots with either transmit setting, (and when presumably stationary), that would indicate that the VSO setting in the SE2 clearly isn't working as it should.

Very inconclusive tests to be honest… the only thing that remained consistent was that SE2 was always visible on a flight tracking app (even when not visible on PAW). I know for sure that in principle (and probably in most cases) I do see SE2 from flying aircraft. But what happened here will still remain a mystery.

That is entirely possible. You must remember that flight tracking apps use multiple receivers with high-gain antennas to pick up 1090 Traffic Signals from multiple directions. These are then combined before being presented on the app, so the app can often report weak transmissions where the signal might be obscured from an individual PilotAware, SE2, (or any other aircraft-based receiver) by metal or carbon fibre, human body tissue or any other non-RF-friendly components in a specific aircraft equipped with PAW, SE2, or any other  receiver. This is why we strongly advocate the use of externally mounted antennas wherever possible (which of course isn't possible with SE2).

Best Regards

Peter